BACILLUS PROTEUS VULGARIS. 473 like the colon bacteria in shape, others are found as very long filaments, and occasionally sporulina-forms are met with. True spirilla-forms are never found. All the forms mentioned may be met with in cultures of the same organism. The diameter of the bacillus is usually about 0.6 //, but the length varies from 1.2 p. or less to 4 /JL or more. No spores are formed. The organisms are actively motile. The long filaments frequently form loops and tangles. Flagella are present usually in large num- ber; upon one of the longer bacilli as many as one hun- dred have been counted. Involution-forms are frequent in old cultures. The bacilli stain well by the ordinary methods. Gram's method is irregular in action, but usually fails to color the bacteria. Upon gelatin plates a typical phenomenon is observed in connection with the development of the colonies, but for the most advantageous observation the gelatin used for making the cultures should contain only 5 per cent, of gelatin instead 10 per cent, as ordinarily used. Kruse1 describes the phenomenon as follows: uat the temperature of the room rounded, saucer-shaped depressions, with a whitish central mass surrounded by a lighter zone, are quickly formed. Under low magnification the center of the growth is seen to be surrounded by radiations extend- ing in all directions into the solid gelatin, and made up of chains of bacilli. Between the radiations and the granular center motile bacteria are seen in active motion. Upon the surface the colon}7 extends as a thin patch, consisting of a layer of bacilli arranged in threads, sending numerous projections from the periphery. Occa- sionally filaments are found in the surroundings. Under certain conditions the wandering of the processes can be directly observed under the microscope. It depends not only upon the culture-medium, but, in part, upon the culture itself. Entire groups of bacilli or single threads, by gradual extension and circular movement, detach themselves from the colony and wander about upon the 1 Fliigge's Mikroorganismen.